118 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



basal segment containing the conventional 11 fibers and a much 

 longer terminal filament consisting of the two central fibers and 

 the limiting membrane, with a small amount of matrix material. 

 Whether this distal filament is actively motile or not has not been 

 determined. The basal body of the flagellum lies immediately 

 below the cell surface and approximately parallel to it; its distal 

 end is marked by a transverse septum. 



No rhizoplasts or other fibrous connectives associated with the 

 basal bodies are detectable. The small nucleus, generally lateral 

 to the kinetosome, contains a poorly-defined nucleolus and is 

 surrounded by a double envelope. 



The chloroplast, occupying a large part of the cell's volume, is 

 roughly hemispherical, with a large eccentric pyrenoid surrounded 

 by a thin shell of clear material believed (Manton and Parke, 1960) 

 to contain starch. Between this shell and the plastid surface, 

 lamellar discs in loose, discontinuous, sometimes overlapping 

 stacks run circumferentially. This arrangement is rather different 

 from the continuous longitudinal orientation of lamellae in 

 chrysomonad chloroplasts, and first aroused Manton's suspicion 

 of the accepted taxonomic position of the species. A subsequent 

 pigment analysis revealed the presence of chlorophyll b, which is 

 unknown in the Chrysophyceae but characteristic of green algae. 



M. pusilla has a single, sausage-shaped mitochrondrion located 

 near the anterior borders of chloroplast and nucleus, adjacent to 

 the kinetosome. In Manton's preparations the internal mito- 

 chondrial membranes are sparse, but they resemble cristae rather 

 than microtubules. A group of small rounded vesicles always 

 seen close to the kinetosome is identified as the Golgi zone; a 

 small fat droplet generally is found nearby. Other larger, empty 

 vesicles occur here and there. The organelles pack the tiny cell 

 so completely that the cytoplasmic matrix is reduced to a minimum. 

 The cell is naked, limited only by one unit membrane. Micro- 

 graphs of cells in division show that the basal body of the flagellum 

 probably divides first, followed by the chloroplast, in which a 

 cleavage furrow forms and progressively constricts the organelle 

 through the pyrenoid, and then by the mitochondrion, which 

 becomes U-shaped before fission. 



As Manton points out (1959a, p. 329), "By the time that a cell 

 is restricted to one nucleus, one plastid, one mitochondrion, one 



